1. Field of the Invention
This invention generally relates to improvements in blind fasteners and the joints produced therewith and, more particularly, to improved blind fasteners with washers.
2. Description of the Prior Art
Advanced composite materials have recently been gaining increasing usage in various aerospace applications because they offer potential weight savings and increased stiffness. The anisotropic properties of composite materials present a unique opportunity to optimize certain structural designs such as elongated aircraft stringers and ribs. In order to form acceptable joints of relatively thin sheet composite materials, conventional blind fasteners have not proven to be completely acceptable.
One reason is the low allowable bearing stress of composite materials which means that the full tensile strength of the blind fastener may not be realized because of bearing failure of the composite material. Present blind fasteners have a relatively limited blind side bearing area and typically expand to approximately 1.3 times the original sleeve diameter. Another reason is that composite materials are weaker in shear, thus causing any holes to require greater edge distances and more area buildup to develop full efficiency of the composite material.
Advanced composite materials are constructed of a resin matrix which has a tendency to crack when holes in the material are expanded, such as by cold working or by an interference fit fastener. Rivets which swell when upset may also crack the resin matrix. Finally, fasteners which form against the blind side composite material surface tend to damage the relatively soft material surface by digging or gouging out some of the material during the bearing surface formation process. Additionally, fasteners which form against the blind side composite material surface may cause stress concentrations during the formation process which exceed the compressive strength limit of such materials, thus causing damage.
Some blind fasteners taught by the prior art form a bearing surface on the blind side of the workpiece before contact is made with the workpiece. However, such fasteners have not proven entirely satisfactory for a variety of reasons, including problems associated with consistently and reliably forming the bearing surface before contact with the workpiece. Additionally, some conventional blind fasteners cannot conform to the irregular contour of the composite material blind surface and tend to crush the raised points of the material, thus reducing overall material strength.
A prior art design for a blind fastener providing an enlarged bearing area is disclosed in U.S. Pat. No. 4,579,491 issued Apr. 1, 1986 to Kull. In Kull, a sleeve and a coil washer expanded over a tapered nose to form a bearing surface. While the fastener does provide a large bearing surface and is insensitive to variations in grip length, the core pin of this fastener is necessarily smaller in diameter than may be preferred, therefore exotic high strength materials are sometimes required to provide this fastener with a preselected strength.
Strobel U.S. Pat. No. 4,832,548, discloses another prior art blind fastener assembly suitable for use in joint assemblies including advanced composite material workpieces. This blind fastener assembly has a core pin, an expander, and a sleeve, each having an initial diameter which permits it to pass through a hole in a workpiece. The core bolt has a head at one end and a shank extending therefrom. The sleeve has a primary portion and a secondary portion, the secondary portion being provided with a frustoconical region interconnected with the main portion by a circular web of material. The sleeve is fitted over the shank of the core pin with the primary portion abutting the head of the core pin. The expander is provided with a tapered nose portion at one end and a head at its other end. The expander is mounted to the shank with the nose portion abutting the secondary portion of the sleeve.
In use, this blind fastener assembly is passed through a hole in a workpiece, or aligned holes in two or more workpieces, until the head of the expander abuts the access side of the joint. Relative longitudinal movement of the core pin and the expander forces the tapered nose into the sleeve and causes it to expand. As the sleeve expands, the web between the primary and secondary portions of the sleeve shears. When the secondary portion of the sleeve abuts the blind side of the workpiece, the continued relative longitudinal movement of the core bolt and the expander causes the primary portion of the sleeve to expand over the frustoconical region of the secondary portion of the sleeve until the primary portion also abuts the blind side of the workpiece. Furthermore, in the preferred embodiment, an expandable washer is mounted to the shank of the core bolt between the sleeve and the expander. The washer is expanded over the tapered nose of the expander ahead of the sleeve such as to form a protective bearing surface between the respective sleeve portions and the workpiece, thereby further minimizing potential damage to the workpiece material during the formation of the joint.
However, this design suffers from several deficiencies. For example, a certain amount of load is necessary during installation in order to shear the web of this design so that the sleeve shears into two pieces. But inconsistent installation and performance problems may result since consistent web shear cannot be achieved (i.e. the web does not shear in exactly the same place with respect to the nose portion of the expander with each installation). The thickness of the web varies because of manufacturing tolerances, thereby causing variation in the shear load. This is a significant problem with smaller sizes. In some cases, the web may not shear until the washer and sleeve assembly contact the workpiece, which increases the washer forming load. In addition, uneven shearing may result if the web thickness is not concentric or of uniform thickness, thereby allowing the secondary portion of the sleeve to cock on the expander. In other words, the uneven advancement of the sleeve may cause the web to shear on one side of the expander before shearing on the other side or the shearing of the web may cause the advancement to be uneven. Also, if the web does not shear until the sleeve and washer contact the workpiece, the washer may wrap around the top of the secondary portion of the sleeve which is away from the workpiece, thereby resulting in a folded washer with a reduced bearing contact.
Although U.S. Pat. No. 4,832,548 teaches the use of a type of washer in certain embodiments of the blind fastener assembly disclosed therein, it has been discovered that if this washer is too short or does not extend beyond the cylindrical region of the secondary sleeve portion, such as illustrated in FIG. 5 of the above-mentioned patent, the requisite forming load would be high. If the washer is too long or extends to the primary sleeve portion, such as illustrated in FIG. 6 of the above-mentioned patent, the requisite forming load would also be high because the primary portion is contained. Therefore, the forming load could exceed the failure strain limit, possibly resulting in the cracking or tearing of the washer.
Furthermore, a portion of the web having a ragged or irregular surface remains on the primary portion of the sleeve after the sleeve shears into two pieces. This irregular portion can cut into the washer during or after installation and eventually damage the composite material. Additionally, the sleeve is typically manufactured from stainless steel which is passivated to minimize corrosion. The exposed shear surfaces on both the primary and secondary portions of the sleeve are subject to greater corrosion since they present non-passivated surfaces to the environment. Moreover, a dry film lubricant is commonly applied to the blind fasteners to assist in installation, and the exposed shear surfaces with this design will lack any such lubricant after shearing occurs, thereby increasing the forming load and making installation more difficult. The complexity of the sleeve design with a web also makes it more difficult to inspect and adds to the manufacturing cost. Manufacturing variations in the web have a significant effect on installation reliability. The improved blind fastener with a washer in accordance with the present invention overcomes these deficiencies.